Method of controlling oil tank fires



United States Patent O METHOD OF CONTROLLING OIL TANK FIRES Joe L.Risinger, Maplewood, N. J., assignor to Socony Mobil Oil Company, Inc.,a corporation of New York Application December 11, 1951, Serial No.261,158

2 Claims. (Cl. 169-1) This invention relates to a method of controllingand combatting oil tank lires.

One of the most serious features of oil tank fires is the boil-over,experienced particularly with crude oils and heavier oils. Practicallyany tank used for the storage of oil has a certain amount, varying froma fraction of an inch up to several feet, of water at the bottom of thetank, below the oil. When the contents of the tank are permitted to burnaway, the oil within the tank becomes heated, and when the tank contentshave burned down enough, and the heat of the oil becomes great enough,this water vaporizes, suddenly, giving rise to great quantities ofoil-steam froth and foam which rise up and overilow the tank, and spreadthe fire. It is this feature which gives rise to the dikes usually seensurrounding large oil tanks, particularly those used for the storage ofcrude oil and the heavier oil fractions. The initiation of the fire isusually accompanied by an explosion which damages the tank roof,frequently removing it entirely over at least a portion of the tank.While there exist many devices for permanent attachment to a tank, atthe roof edge, for introducing foamite or similar lire lightingchemicals, these are so frequently damaged by the initial ame burst thatmany tank installations do not use them, but depend rather upon portablefoam towers and the like to apply fire fighting chemicals throughopenings made by the initial blast. Additionally, many crude oil tankfarms are placed in relatively isolated localities, and equipment andman-power, particularly man-power, are not available for lire fightinguntil the lire is quite well established. In such circumstances, even ifa fire is then put out, the upper portion f the sides of the tank areusually so damaged that the tank must be entirely rebuilt, exceptingpossibly the bottom.

This invention is particularly concerned with a novel concept incombatting and controlling such lires, capable of utilizing equipmentwhich is normally installed in the tank for other purposes, whichequipment is seldom damaged or rendered inoperative by the tire untilits very late stages are reached.

This invention is based upon the fact that to support combustion, theoil immediately adjacent the burning surface must be at or above itsflash point, and asecond fact, that the remainder of the oil in the tankis not that heated, taking usually a period of several hours to heat upto any temperature near the combustion supporting level.

This becomes most clear when we consider a tank full of kerosene. TheHash-point of kerosene, the temperature at which a small cup-full ofkerosene will give olf enough vapors to support a combustion upon itssurface, is usually at least 125 F. The normal temperature of the bodyof kerosene in storage in a large tank is usually about the meanatmospheric temperature existing in the locality of storage at the timein question. This temperature is usually of the order of not above70-90" F., even in summer in very hot climates. In a tank ot' kerosene,burning at its surface, only an inch or so of the kerosene, at the top,is at the ash point until after the tank has ICC been burning for a timeof the order of an hour or more. (Of course it is obvious that the fireis usually started by a mechanism involving the presence of vapors in acombustible mixture of vapor and air, which does not necessitate thesurface of the oil being above its flash point.) Opposed to this slightlayer of hot oil, there is much depth, usually many feet, of oil wellbelow the ash point. Now if we agitate the oil suiiciently to bringabout mixing and cooling of the surface oil, it should be possible tostop combustion.

In actual trial, an open-top tank, thirty feet in diameter, containingabout twenty feet of kerosene, was set on fire. After it was burningwell, a stream of compressed air, rather small in volume, was introducedinto the water draw-off line-a small pipe entering the tank near itsbottom, and not extending into the tank beyond the side wall-and withinthirty seconds the lire was out.

In this test the air applied was at a rate of from 40 to C. F. M., at apressure of about 5-7 p. s. i. for a duration of less than a minute.From this may be seen that the air requirements are quite low.

It is apparent that the air requirements set forth above mayalternatively be expressed as an amount varying from about 6 cu. ft. toabout 13 cu. ft. of free air per minute per sq. ft. of oil surface.

In a storage tank containing kerosene, gas-oil, lubricating oils, fueloils, or any oil having a ash point more than a few degrees aboveatmospheric temperatures the same thing will happen Thus it may be seenthat this invention provides a positive and economical method for therapid, safe, and complete extinguishment of lires in tanks containingoils having a flash point above atmospheric temperature.

With gasolines, naphthas, and most crude oils, where the flash point ofthe oil is below the storage temperature, that is, below the averageambient temperature during the period of storage, the method providesonly a form of control. In most crudes there is a suieient gasolinefraction to support combustion. The greater portion of the crude oil(usually at least 75% of it by volume), is high flash material.Consequently the same technique, applied to a crude oil lire, will soconsiderably diminish its activity that it becomes easily susceptible toextinguishment by other methods.

In a test made with crude oil, the same thirty foot tank was filled to adepth of about twenty feet with crude oil, leaving about seven feet ofside wall above the crude. This was ignited, and after it was burningwell, air was injected through the water draw-off line. Within a spaceof 45 seconds the lire had so subsided to only a flickering flame abovethe side wall. Using a spider device to secure air distribution over thebottom of the tank, the fire immediately subsided so that it waspossible to stand at the upper edge of the tank wall and observe thelire icking across the oil surface. This is a thing that to my knowledgehas never before been done with a crude oil re. This lire was allowed toburn for several hours, while under such control, to determine at whatrate the oil in the tank below the re became heated. Using a paint onthe outside of the tank wall which changes color with temperaturechange, it was not possible, through the duration of the test, to findany heating of the oil below the burning surface. In similar tests,without agitation, heat can be seen to be moving downward at anappreciable rate. During this period the introduction of air throughother means, up to and including a rather elaborate layout of perforatedpipe on the bottom of the tank was tested. The conclusion drawn fromthese tests was that the better distribution of air except in crude oiltanks did not usually secure effects enough better to warrant expense ofinstallation and maintenance.

In this crude oil test, the air supply was again at a rate from 40 to 90C. F. M. and at a pressure of from 5-7 p. s. i. Had the desire beensolely to so control the fire as to render it readily extinguishablewith foam, etc., the duration of air supply would have been less than aminute to minimize the ame, plus whatever time would be necessary tosupply and apply the extinguishing agent.

As another point of interest, when a tank of heavy oil has been burningfor some time, and the upper foot or so of the oil is quite well heated,the application of any of the water-borne foam type extinguishingcompounds is very apt to result in the formation of oi1-foam andslopover of burning oil and foam, for the same reasons that causeslop-over when the heat level has reached the bottom water, and withconsequences equally dangerous. Tests have shown that with a tank soheated, the introduction of air will so cool the surface oil, even afterextensive burning, as to permit the introduction of waterborne foamextinguishing compounds without slop-over.

The air agitation of crude oil tanks to prevent boil over has beenpreviously proposed, the proposal being based upon tests in small tanks.When submitted to the industry, it encountered adverse independent testsand this proposal failed of adoption. It has been proposed tomechanically agitate the tank contents to arrive at the same result. Tomechanically agitate the contents of tanks more than a few feet indiameter, (most oil industry tanks are over 100 feet in diameter),requires equipment and investment of a size that makes this proposalabsurd. It has been proposed to pump tank contents from a low level andspray them upon the burning surface. This requires a spray system,widely and evenly distributed over the tank oil surface and floatablysupported to remain at that tank oil surface. quirements for thisequipment likewise render this proposal practically absurd.

In contrast, the method here proposed is of the utmost simplicity, usingonly equipment already installed and present in the tank for otherpurposes.

This may be seen by reference to the drawing which is made a part ofthis specification.

In the single diagrammatic ligure of this drawing, 1 denotes the shellof a tank, shown in section, and containing oil, 2. This tank isequipped with the usual product drawoi line 3, which extends to themiddle of the tank at a point adjacent the bottom thereof. External tothe tank shell, drawoff line 3 is usually equipped with a valve 4, andat some point in the length of pipe 3, beyond valve 4, there will beanother valve 5. For convenience, this valve 5 is here shown as adjacentvalve 4. It is assumed that the oil 2 in tank 1 has been ignited, andhas been burning at its surface. Compressed air has been introducedthrough valved pipe 6, with valve 5 closed and valve 4 open. Passingthrough pipe 3, this air rises through the oil 2 at the point indicatedby character 7, creating an active upward motion of the oil, whichsurges upward at 8 and tlows outward at 9, 9, cooling the surface layerof the oil and causing the flame to retreat ahead of it to the position10, 10, along the tank sides, where the lire is soon chilled out ofexistence.

The tank 1 is also tted, customarily, with one or more water draw-otfconnections, such as that shown at 11. In a large tank there will beseveral of these, located around The investment and maintenance retheperiphery. Each, or all, of such water draw-off connections may also beused for the introduction of compressed air.

This method will work with pools of oil, such as those in open or roofedearthen storage pits, as well as with oil in tanks.

Since any tank or pool in which oil is stored will customarily beequipped with piping leading to a point near the bottom of the oil, foroil handling purposes, and since such piping is customarily open at theoil end and so arranged at points outside the tank or pool that it canbe broken into and' arranged for air introduction, any such piping canbe used. On tank storage, the water draw-olf line is most convenient,since it is a pipe of relatively small diameter, valved outside thetank, and outside the valve, the water drawoff pipe ends in an open end,quite convenient for attaching to a compressed air supply. Since inalmost any location, portable gasoline-motor driven compressors are athand or procurable, the method is adaptable to widespread use.

I claim:

l. That method of extinguishing a combustion upon the surface of astored body of oil, which oil has a ash point above atmospherictemperature, which comprises introducing into the oil, at a point wellbelow the surface thereof, at least one stream of compressed air, theamount of such air being from about 6 cubic feet to about 13 cubic feetof free air per minute per one hundred square feet of oil surface, thepressure of such air being sufficiently above atmospheric pressure toinsure ready entry of air into the oil, whereby submerged portions ofthe oil, existing at a temperature below the ash point of the oil arebrought into contact with and mixed with the surface portion of the oilto cool the surface below the ash point and extinguish the combustion bydepriving it of oil vapors.

2. The method of extinguishing a combustion upon the surface of a storedbody of oil which comprises introducing into the oil, at a point wellbelow the surface thereof, at least one stream of compressed air, theamount of such air being from about 6 cubic feet to about 13 cubic feetof free air per minute per one hundred square feet of oil surface, thepressure of such air being suiciently above atmospheric pressure toinsure ready entry of air into the oil, whereby submerged portions ofthe oil body are brought up to and admixed with the surface thereof tocool said surface oil, diminish the evolution of vapors therefrom andlessen the combustion, and thereafter applying a fire extinguishingagent to the surface of said body of oil, by which two-step method thecombustion may be extinguished in a shorter time and with less lireextinguishing agent than had the agent alone been used.

References Cited in the le of this patent UNITED STATES PATENTS AitkenMar. 20, 1951 OTHER REFERENCES

1. THE METHOD OF EXTINGUISHING A COMBUSTION UPON THE SURFACE OF A STOREDBODY OF OIL, WHICH OIL HAS A FLASH POINT ABOVE ATMOSPHERIC TEMPERATURE,WHICH COMPRISES INTRODUCING INTO THE OIL, AT A POINT WELL BELOW THESURFACE THEREOF, AT LEAST ONE STREAM OF COMPRESSED AIR, THE AMOUNT OFSUCH AIR BEING FROM ABOUT 6 CUBIC FEET TO ABOUT 13 CUBIC FEET OF FREEAIR PER MINUTE PER ONE HUNDRED SQUARE FEET OF OIL SURFACE, THE PRESSUREOF SUCH AIR BEING SUFFICIENTLY ABOVE ATMOSPHERIC PRESSURE TO INSUREREADY ENTRY OF AIR INTO THE OIL, WHEREBY SUBMERGED PORTIONS OF THE OIL,EXISTING AT A TEMPERATURE BELOW THE FLASH POINT OF THE OIL ARE BROUGHTINTO CONTACT WITH AND MIXED WITH THE SURFACE PORTION OF THE OIL TO COOLTHE SURFACE BELOW THE FLASH POINT AND EXTINGUISH THE COMBUSTION BYDEPRIVING IT OF OIL VAPORS.